Acetoacetylated polymers are of intense current interest. Incorporation of acetoacetate groups into polymers generally affords several benefits, including reduction of solution viscosity (valuable in coatings applications) and introduction of a handle (the acetoacetate group) for ready polymer crosslinking. The crosslinking chemistry of the acetoacetate group is rich and growing. These attributes make acetoacetylated polymers of particular interest in thermoset coatings applications.
Polymers with affinity for water are also currently of great interest. Water-soluble polymers can be used to change the rheology of aqueous solutions, and so find many industrial applications, for example in the food industry as thickeners, in the oilfield industry as suspension aids, in the pharmaceutical industry as excipients, and in the coatings industry as film formers.
Cellulose is a readily available, naturally occurring polymer, whose derivatives are highly valued in coatings applications as well as the other industrial applications cited above. It is clear, therefore, that the ability to efficiently acetoacetylate cellulose might afford products of commercial interest. In particular, if water-soluble products could be made in this way, materials of great interest in industry might result.
There is relatively little literature on the acetoacetylation of cellulose. P. J. Wiezevich and A. H. Gleason (U.S. Pat. No. 2,103,505, 1937) taught a method of acetoacetylation with diketene (which they incorrectly called "cyclobutane 1,3-dione") which they applied to cotton linters in one of their examples. They claimed cellulose acetoacetate by reaction of diketene with cotton linters employing "an acid esterification catalyst", but gave no supporting details about either the method or the product.
H. Staudinger and T. Eicher (Makromol. Chem., 1953, 10, 261-279) teach complete acetoacetylation of cellulose employing a heterogeneous system of regenerated cellulose in acetic acid with sodium acetate catalyst, and diketene as acetoacetylating reagent. They obtained a product with a degree of substitution ("DS") per anhydroglucose unit ("AGU") of 3.0, as measured by elemental analysis. The product was soluble in acetic acid and acetone, and insoluble in water.
V. H. Reblin expanded upon these results (Chimia, 1968, 22, 230-233). He treated cotton with diketene, according to the method of Staudinger and Eicher. This gave a partially acetoacetylated cotton, still water insoluble, which was subjected to crosslinking with aliphatic diamines. Reblin found that the crosslinking gave bis(enamine) linkages, and dramatically decreased the tensile strength of the cotton. The crosslinks could be hydrolyzed with dilute acids.
None of these prior art teachings allow direct, homogeneous acetoacetylation of cellulose to the desired DS. None of them give evidence of preparing a homogeneously substituted CAA with DS/AGU less than 3.0. Nothing known in the prior art teaches preparation of a water-soluble cellulose acetoacetate. Nothing known in the prior art teaches acetoacetylation of cellulose in the absence of acid or base catalysis.
Most of the known cellulose solvents are unsuitable for reactions in which cellulosic hydroxyls serve as nucleophiles. The solvent system recently introduced by C. L. McCormick (U.S. Pat. No. 4,278,790, 1981) and A. F. Turbak, A. El-Kafrawy, F. W. Snyder, Jr., and A. B. Auerbach (U.S. Pat. No. 4,302,252, 1981) are exceptions in that they do permit certain such reactions. McCormick and Turbak et al. have taught that cellulose may be dissolved in solutions of lithium chloride in either N,N-dimethylacetamide or 1-methyl-2-pyrrolidinone. McCormick has further taught (U.S. Pat. No. 4,278,790; Polymer, 1987, 28, 2317-2323) that electrophilic reagents may be added to these cellulose solutions to derivatize cellulose. McCormick has prepared cellulose acetate, methyl cellulose, cellulose carbamate, and other derivatives (but not CAA) in this way.
There is, therefore, a need for a process by which CAA of the desired degree of substitution may be prepared directly from cellulose. It is desirable that cellulose be reacted in solution, to assure homogeneous substitution along the polymer chains. The process should be economical and amenable to scale up to industrial production. It would be desirable to have available from this process a CAA of the correct composition, such that it would have good solubility in water.